Process for copolymerizing conjugated dienes with unsaturated nitriles in aqueous emulsion



Patented June 17, 1952 PROCESS FOR COPOLYMERIZING CONJU- GATED DIENES WITH UNSATURATED'NI- TRILES IN AQUEOUS EMULSION Harold F. Park, East Longmeadow,- Mass., as-

sig nor to Monsanto Chemical Company, St.

-Louis, Mo., a corporation of Delaware No Drawing. Application May 12, 1947, Serial N0. 747,594

' 1 Claim. 1

This invention relates to the copolymerization of conjugated dienes. More particularly, the invention relates to the preparation of diene-acrylonitrile copolymer emulsions.

The emulsion copolymerization of conjugated dienes with ail-unsaturated nitriles, especially acrylonitrile, is an important process in the production of synthetic rubber. The-conventional processes have included thesteps of copolymerizing the monomeric materials in aqueous emulsion, coagulating the emulsion and washing the coagulated material to remove emulsifying agent, catalyst, coagulant, etc. The coagulating and washing steps are cumbersome, time-consuming and add materially to the cost of preparing the copolymer.

It is an object of this invention to provide a process for the copolymerization of conjugated dienes with B-unsaturated nitriles in aqueous emulsion.

A further object is to provide a process for preparing aqueous emulsions of conjugated dienea, 3-unsaturated nitrile copolymers which may be direct dried.

Still another object is toiprovide a process for copolymerizing butadiene and acrylonitrile in aqueous emulsion such that the emulsion produced may be direct dried without being coagulated and washed.

These and other objects are attained by copolymerizing a conjugated diene with an l p-unsaturated nitrile in aqueous emulsion using critical ranges of emulsifying agent, catalyst, pH regulator, modifying agent, etc., as will be more particularly set out.

The following examples are given in illustration and are not intended as limitations on the scope of this invention. Where parts are mentioned, they are parts by weight.

Example I 0.4 part of the sodium sulfonate of dioctyl succinate were dissolved in 200 parts of water in an autoclave. To this solutionwere added 0.02 part of potassium persulfate, 0.024 part of sodium bicarbonate, 0.03 part of sodium bisulflte, 0.2 part of dodecyl mercaptan and 40 parts of acrylonitrile. The mixture was then cooled to about 5 C. and 65 parts of purified butadiene were added. The butadiene was allowed to boil until about 5 parts had evaporated to remove dissolved and occluded air. The autoclave was then sealed and the temperature raised to 30 C. The temperature of the reaction was maintained. at 30 C. for about 30 hours during-which thereactants were subjected (Cl. ZED- 82.7)

2 to constant agitation. 'Theautoclavewasthen opened and 2 parts of phenyl B-naphthylamine were added. The'resultant emulsion could be dried directly to yield a high grade-solvent-resistant synthetic rubber having good moisture resistanceand excellent physical properties.

Example I I 2 parts of diisobutyl sodium sulfonate and 1 part of potassium persulfate were dissolved in 1800 parts of boiled distilled water. The pH of the solution was adjusted to about 6.5 with potassium hydroxide. 125 parts of fumaronitrile, 4 parts of carbon tetrachloride and 375 parts of purified isoprene wereadded to the solution. The mixture was then heated to reflux temperature at atmospheric pressure and maintained at reflux with constant agitation until the reflux temperature rose to about C. The emulsion thus produced was cooled to room temperature and 3.5 parts of hydroquinone were added. The emulsion could be dried directly without coagulation to produce an elastomer having excellent physical properties and good solvent and water resistance. The elastomer could be compounded and fabricated by conventional procedures without difficulty.

Prior processes for the preparation of copolymers of conjugated dienes and c p-unsaturated nitriles in aqueous emulsion have required large amounts of emulsifying agents and catalysts which, if left in the'synthetic rubber, are extremely detrimental to the physical properties and age resistance thereof. 'It has now been found that extremely smallquantities of emulsifying agent and catalystmay be used and subsequently allowed to remain in the synthetic rubber produced, providing certain conditions are maintained.

In the first place, the emulsifying agent must be one of the so-called ionic compounds which is capable of spontaneous separation into two electrically charged portions, one of which is solvated to a greater extent by the dispersed phase of the emulsion than the other so that the dispersed particles will acquire an 'eleetrical charge and thus repel one another to form a stable emulsion.

The emulsifying agent must also be surfaceactive enough to minimize the surface-energy change resulting from change in the interfacial surface area. Emulsifying agents which regulate the interfacial tension between 0 and 20' dynes per centimeter meet this requirement.

The emulsifying agent must be so solvated by the continuous phase that the amountofl emulsifying agent absorbed at the dineric interface will buffer the impact of colliding particles and thus decrease the coalescence tendency. It must also structuralize the continuous phase so that cybotaxis will reduce to a minimum the tendency of this phase to disperse. The emulsifying agent must also increase the viscosity of the external phase in order to diminish the magnitude and velocity of kinetic and agitational movement.

The following emulsifying agents will meet the above stated conditions provided that they are used in an amount within the range of .05 to 1.0 part per 100 parts of monomer mixture and provided that other critical conditions pertaining to catalyst, pH regulator, polyvalent ion, agitation and ebullition as set forth below are met: soaps including alkali metal salts of long chain fatty acids such as lauric acid, palmitic acid, stearic acid, oleic acid, coconut oil fatty acids, polyacrylic acids, polymerized alpha-alkyl acrylic acids, styrene-maleic anhydride copolymers, etc.; quaternary ammonium salts including diethyl amino ethylol ethyl amine hydroacetate, cetyl dimethyl benzyl ammonium chloride, mono amino meta diphenyl benzene stearate, etc., ammonium or alkali metal salts of sulfonated organic compounds, including sodium salts of alkylated aryl sulfonates, such as dibutyl phenyl phenol sodium disulfonate, monobutyl phenyl phenol sodium monosulfonate, monoethyl phenyl phenol potassium monosulfonate, sodium salts of alkyl naphthalene sulfonic acids, isobutyl naphthalene sodium sulfonate, isopropyl naphthalene sodium sulfonate, sodium salts of sulfonated hydrocarbons, sodium salts of alkyl polyether sulfonates, sodium salts of sulfonated lignin, sodium tetrahydronaphthalene sulfonate, etc. In place of the sodium salts, ammonium salts, or salts of other alkali metals such as lithium, potassium, rubidium, and cesium may be used. A mixture of emulsifying agents may be used providing that they are taken from the same class.

As a further'means for maintaining the critical features of the emulsifying agents at optimum efficiency, a compound which operates to regulate the pH of the emulsion may be used in the proportions from about .003 to about 0.5 part per 100 parts of monomer mixture. For such emulsifying agents as the alkali salts of sulfonated organic compounds or the quaternary ammonium salts, the pH of the emulsion should be regulated to from about 4.5 to about 6.5. Examples of compounds which operate to maintain a pH of from 4.5 to 6.5 are sodium bicarbonate, disodium hydrogen phosphate, sodium acetate, sodium citrate, potassium formate, etc. In the event that soaps are used as emulsifying agents, the pH of the emulsion may be regulated between about 8 and about 11, using such alkaline materials as hydroxides of alkali metals and alkaline earth metals and quaternary ammonium compounds. The amount of pH regulator used will depend somewhat on the amount and type of catalyst used and especially on the pH of the decomposition products of the catalyst. If hydrogen peroxide is used as a catalyst, the amount of pH regulator may be as small as .002 part per 100 parts of monomer mixture. The amount of pH regulator may also be kept at a minimum if the copolymerization is carried out in the absence of air, e. g., under an atmosphere of carbon dioxide or nitrogen. In many combinations it will not be necessary to use a pH regulator.

Other catalysts than the potassium persulfate shown in the examples may be used, such as sodium perborate, hydrogen peroxide, acetyl peroxide, ammonium persulfate, ceric sulfate, etc. The catalysts must be water-soluble and must have an oxidation reduction potential of less than -1.5 volts. Such well known catalysts as benzoyl peroxide, lauroyl peroxide, di(tertiary butyl) peroxide, etc. are, for practical purposes, insoluble in water and cannot be used, even though their oxidation reduction potential is less than -1.5 volts, since their use in the present process produces polymers greatly inferior to those made with the preferred catalysts. The amount of catalyst used may be varied between about .005 to about 0.4 part per parts of monomer mixture. Use of water-insoluble catalysts or catalysts having an oxidation reduction potential of more than 1.5 volts or use of the preferred catalysts in amounts outside of the critical range will destroy the conditions which make it possi; ble to use the emulsifying agents according to the process of this invention.

Another condition which must be met before emulsions which may be direct dried can be made is the relative absence of polyvalent ions. If the emulsifying agent belongs to the class known as anion-active, the concentration of polyvalent cations such as aluminum, chromium, ferric, ferrous, calcium, barium, strontium, magnesium, zinc, etc., ions must be less than 100 p. p. m. If the emulsifying agent is cation-active, the concentration of polyvalent anions must also be kept below 100 p. p. m. In addition, other materials capable of destroying active centers must not be present in a concentration exceeding 0.01 molar based on total monomer. Examples of such impurities are molecular oxygen, iodine, sulfur, bromine, selenium, phenols, quinones, amines, etc.

In order to successfully produce emulsions with the emulsifying agents and proportions shown above, the system should be subjected to mechanical agitation slightly in excess of that required to provide the increase in surface energy necessary to effect dispersion of the discontinuous phase to an average particle size of between about 0.05 and 0.5 micron. However, care must be taken to prevent suflicient agitation to cause surface turbulence. Furthermore, vigorous ebullition must be avoided since it increases agitation above permissible limits and also since the formation of bubbles of vapor creates a gas-liquid interface which will absorb a portion of the surface-active emulsifier which portion then becomes unavailable to the monomer and stable emulsions cannot be obtained. 7

If desired, 'a modifying agent such as the dodecyl mercaptan shown in Example I or the carbon tetrachloride shown in Example II may be used. In place of the compounds already shown, various other modifying compounds such as mercaptans, thiols, organic sulfides, thio and dithioic acids and their derivatives, xanthogenic acids and their derivatives, thiocarbamic acids and their derivatives, etc. may be used. Moreover, mixtures of two or more of the modifiers may be used. The amount of modifying agent may vary between about 0.025 and about 0.35 part per 100 parts of monomer.

The dienes which may be used to form the copolymers of this invention are conjugated dienes such as butadiene, isoprene, chloroprene, cyclopentadiene, dimethyl butadiene, etc. A combination of two or more dienes may be used.

The nitriles of the examples 'may be replaced in whole or in part by other polymerizable ,5- unsaturated nitriles such as the a-alkyl, a-aryl, aaralkyl and a-cycloalkyl substituted acrylonitriles, nitriles of B-unsaturated acids such as maleic, fumaric, mesaconic, citraconic, itaconic, methylene malonic, etc. acids. The ratio on a weight for weight basis of diene monomer to afiunsaturated nitrile may be varied between 1:10 and 10:1. The preferred range lies between 1:5 and 5:1.

In making the emulsions, the ratio of monomer mixture to water may be varied from about 1:10 to about 1:1. This ratio is not critical, but for ease in recovering the copolymer and other economic reasons, the ratio of one part of monomer mixture to about two parts of water is preferred. In making the emulsions, all of the ingredients may be mixed together at the same time, followed by polymerization, or they may be separated into a plurality of portions which are then mixed over an extended period of time. A preferable procedure is to add all of the emulsifying agent to the greater proportion of the water and then add a monomer mixture plus a modiher, if desired, to the heated solution of emulsifying agent at a rate slow enough to permit easy control of the temperature of the reaction. The catalyst, dissolved in water, may be added continuously or at intervals during the addition of the monomer mixture, providing that a portion of the catalyst must be added at the same time as, or prior to, the first addition of the monomer mixture.

The temperature of the reaction may be carried out at the reflux temperature or at temperaabove the reaction has been supplanted by vapors of the volatile diene.

The process of this invention is particularly advantageous since it makes possible the emulsion copolymerization of polymerizable dienes with O B-unsaturated nitriles to produce synthetic rubber latices from which the synthetic rubber may be easily and quickly obtained by direct drying methods such as drum drying, oven drying, spray drying, etc. The process makes it unnecessary to use large amounts of emulsifying agents; it makes it unnecessary to coagulate the emulsion since it leaves substantially no undesirable impurities in the synthetic rubber which must be washed out before the rubber can be further processed.

It is obvious that many variations may be made in the processes and products of this invention without departing from the spirit and scope thereof as defined in the appended claim.

What is claimed is:

A process which consists of copolymerizing 65 parts of butadiene with 40 parts of acrylonitrile in an aqueous emulsion containing 0.4 part of the sodium salt of dioctyl succinate, 0.02 part of potassium persulfate, 0.024 part of sodium bicarbonate, 0.03 part of sodium bisulfite and 0.2 part of dodecyl mercaptan, said polymerization being run at 30 C. for 30 hours in the absence of substantial amounts of oxygen.

HAROLD F. PARK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,227,517 Starkweather et al. Jan. 7, 1941 2,234,204 Starkweather et al. Mar. 11, 1941 2,333,403 Youker Nov. 2, 1943 2,375,140 Semon May 1, 1945 2,387,385 Clifford Oct. 23, 1945 2,409,915 Vanderbilt et al. Oct. 22, 1946 2,473,390 Rose et al. June 14, 1949 

